In recent years, with the rapidly growing of portable and cordless electronic devices, there has been advancement in putting, as a power source for driving these devices, non-aqueous electrolyte secondary batteries having a high voltage and a high energy density into practical use. Technical development of non-aqueous electrolyte secondary batteries has been accelerated not only for small-sized consumer products but also as large-sized batteries having a large capacity for the purpose of energy storage, electric vehicles and the like. For example, there has been a progress in applying non-aqueous electrolyte secondary batteries to high-output purposes such as electric tools and hybrid electric vehicles (HEV) by contriving the electrode structure and the current collector structure of the same.
The non-aqueous electrolyte secondary battery as mentioned above has a wound-type electrode group comprising belt-shaped positive and negative electrodes each composed of a material mixture layer and a core member, and a separator interposed between the positive electrode and the negative electrode. The separator has a role of electrically insulating the positive electrode and the negative electrode while retaining the non-aqueous electrolyte, and a sheet of a microporous thin film mainly composed of polyethylene having a thickness of several tens of μm is used as the separator.
Several contrivances can be thought of in applying the non-aqueous electrolyte secondary battery into high-output purposes as described in the following. The thickness of the positive electrode and the negative electrode is rendered smaller and the surface area thereof is enlarged. Further, in each of the positive electrode and the negative electrode, an exposed part of a core member is formed along one side which is parallel to the longitudinal direction of the electrode. For example, in the wound-type electrode group, an exposed part of the positive electrode core member is disposed at one end face in the winding axis direction and an exposed part of the negative electrode core member is disposed at the other end face. A positive electrode current collector terminal and a negative electrode current collector terminal are connected to the exposed part of the positive electrode core member and the exposed part of the negative electrode core member, respectively. In this manner, an electron transmission path can be secured thoroughly in the belt-shaped electrode and therefore the high-output characteristics can be improved.
In general, since the non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have a higher energy density per unit volume as compared to nickel-metal hydride storage batteries and lead acid batteries, the technique for assuring the safety is critical. For example, in order to prevent occurrence of internal short circuits, a proposition has been made to form a porous film on the surface of either the positive electrode material mixture layer or the negative electrode material mixture layer (see Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Publication Hei 7-220759 (Patent Publication No. 3371301)